ML21230A357

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LBB Studies V4
ML21230A357
Person / Time
Issue date: 08/18/2021
From:
NRC/RES/DE
To:
Homiack M
Shared Package
ML21230A354 List:
References
Download: ML21230A357 (11)


Text

Leak-Before-Break Studies NRC Phase 1 LBB Report/ Plans for LBB Phase 2 and EPRI Report August 18, 2021 1

Introduction 2

- Application of xLPR to a real-life problem: Impact of primary water stress-corrosion cracking (PWSCC) on a Reactor Vessel Outlet Nozzle (RVON) for a Westinghouse 4-loop Nuclear Power Plant

- Largest number of same weld type with Alloy 82/182 within leak-before-break (LBB)-approved line segments in the U.S. nuclear fleet

- Coordinated effort by NRC/RES and EPRI via a memorandum of understanding, with the following goals:

1.

Develop a defensible strategy of xLPR use for risk-informed decision making 2.

Initiate the first phase of a system-level analysis considering a loop or a whole unit 3.

Test different Quantities of Interest (QoIs) as criteria for decision-making

- Program to be followed by a second phase considering other weld locations and combination of results at the system level

Problem considered 3

  • Reactor Vessel Outlet Nozzle (RVON) for a generic Westinghouse 4-loop pressurized-water reactor

- 34 inches diameter (0.869 m)

- 2.6 inches thickness (0.0663 m)

- Temperature : 320.5°C

- Pressure 15.41 MPa

- Focus on circumferential cracks (axial cracks considered in sensitivity run)

- Simulation time : 80 years (timestep : 1 month)

- Large number of uncertain inputs (e.g.,

welding residual stresses (WRS), crack initiation parameters, crack growth parameters)

Quantities of Interest 4

  • Probability of rupture with leak rate detection
  • Probability of Leak Rate Jump: going from less than 10 gpm to more than 50 gpm in one timestep
  • Probabilistic LBB ratio (critical crack size over detectable leakage crack size)

- Similar to the deterministic ratio in NRC Standard Review Plan Section 3.6.3

- Estimated for each realization leading to a distribution of the LBB ratio

  • Distribution of time from detectable through wall crack to rupture Outputs in blue are directly estimated in xLPR - other outcomes need postprocessing but can be done easily in Excel

Probabilistic Approach 5

- Considered acceptance threshold is 10-6 occurrence per reactor year.

- Equivalent to probability of occurrence of 8 x 10-5 over 80 years

- Sample size considered between 70,000 and 100,000 should lead to respectively 5 to 8 events on average, if the probability is higher than the threshold

- Analysis conditional on having a crack at time 0 use sample size of 5,000 since probability of having crack over 80 years is in the 10-3 range

- Large amount of data saved, so a single probabilistic run was not practical

- 7 replicates of 10,000 to 15,000 sample size, each with different random seeds, were run and combined

- Crack leak rate, inner length, outer length, and critical ratio extracted to estimate the composite QoIs

Reference results (1/2) 6

- Probability of leak rate jump and rupture with leak rate detection too low to occur over 100,000 realizations

- Good on a regulatory standpoint since it shows the risk is below the acceptable range. Not ideal on a scientific standpoint as there is no quantification of the risk

- Rather, alternate QoIs are presented (P(1st crack),

P(1st leak) and P(rupture) without inspection or leak rate detection)

Reference results (2/2) 7 Composite QoIs give results for different criteria (for instance 1 gpm and 10 gpm leak rate detection limits)

Results are conditional on having pipe rupture and do not reflect this probability 10 gpm 1 gpm LBB ratio Time between detectable leak rate and rupture 10 gpm 1 gpm

Sensitivity Case: Impact of WRS Profile (1/2) 8

  • New WRS mean profile with similar uncertainty applied
  • Larger mean at insider diameter leads to higher probability of 1st crack
  • Lower minimum reduces probabilities of 1st leak and rupture relative to the reference value Still, no ruptures with leak rate detection or leak rate jump events

Sensitivity Case: Impact of WRS Profile (2/2) 9

  • Stronger impact on LBB ratio and LBB time lapse than for the reference case
  • Use of multiple metrics gives more insights on scenario differences and increases confidence in the results

Sensitivity Case: Impact of Temperature 10

  • Change in operating temperature (deterministic value) from 320.8°C to 330°C
  • Considered large change since the same temperature is applied through the whole 80-year simulation time
  • Temperature affects initiation and growth, thus impact in all three metrics presented Still, no ruptures with leak rate detection or leak rate jump events

Conclusions and Perspectives 11

  • This study helped identify bugs and enhancements for xLPR (they will be fixed in Version 2.2) but overall, the code is ready to be used for regulatory decisionmaking
  • Beyond the already available outputs, composite QoIs can be defined and calculated in post-processing without too much effort
  • This study helped identify advantages and drawbacks for each QoI considered and showed the benefit of using multiple metrics
  • This first analysis serves as a good basis to extend the process to other weld types (pressurizer surge line, reactor coolant pump ) and estimate risk at the system level